Sen. Arlen Specter is leaving the door open to the possibility of running as an independent next year, a move that would add a layer of intrigue to what is shaping up as the closest watched Senate race in the country.

He suggested to The Hill he would do it only as a last resort, with the understanding that he would caucus with Republicans -- similar to the move Sen. Joseph Lieberman made after changing his party affiliation in 2006.

“It’s pretty hard to run without a party,” Specter told The Hill. “It’s always something that could be a possibility. But then I wouldn’t be in the Republican caucus — wouldn’t have quite the standing as a Republican.”

A party switch -- to unaffiliated -- would set up a potentially titantic clash, with Club for Growth president Pat Toomey the likely favorite to win a GOP primary and with Democrats likely to field a strong candidate in the hope of pulling off a win.

State laws prevent candidates from running as independents after the primaries next spring, meaning Specter would have to join the race as a third-party candidate beforehand.

Specter has ruled out joining the Democrats, despite a push from Gov. Ed Rendell.

“I’m staying a Republican because I think I have a more important role to play there,” he said. “I think the United States very desperately needs a two-party system. … And I’m afraid that we’re becoming a one-party system, with Republicans becoming just a regional party.”